Cost basis
What you paid for an investment, including reinvested dividends. Your taxable gain or loss when you sell is the sale price minus your cost basis.
Your brokerage tracks cost basis for you and reports it on Form 1099-B when you sell. Reinvested dividends add to your cost basis, because you already paid tax on them.
Keeping your own records matters most for investments you moved between firms and for crypto, where you may need to prove what you paid.
Example
Buy shares for $4,000 and sell them for $5,200, and your gain is $5,200 − $4,000 = $1,200.